Electrohub CG

[Balu]

Having to be on the throttle stick all the time is quite common to multirotors. Sometimes you can hit the "sweet spot", but usually you have to adjust the throttle all the time - unless you have a barometer and an "altitude hold" setting in your flight controller.

Self leveling just means your copter will come back to level if you let go of the sticks. It will not stop moving, but still drift in the direction you were flying. It will not hold altitude unless your flight controller has an option to do so .

That "jumping" from the ground is something a lot of us experience. I just try to cut the throttle at the right moment

 

[SidneyKi]

Okay,..

Thanks Balu

 

[JourneyFPV]

Hello,

I have finish my electrohub build with the flip 1.5.
My first flight test was not a success. The MultiRotor tip forward and broke one of the propeller.

I just received the replacement propeller and I was testing it slowly by adding power to the electrohub a little at a time until it almost takes off. It will turn a little and tip forward some before almost taking off.

I wanted to test if it is out of balance or CG is off. Is there a CG on the electrohub?
I have the camera/battery mount on it, not sure where to adjust the battery location. I do not have a camera on it now.

also I notice one of the motor is hot compare to the other three. Is this normal?

Hi Sidney, My electrohub build is doing the same this as yours - moving forward when i take off. how did you solve this issue. was it by resetting the Giro?

 

[SidneyKi]

Hello JourneyFPV,

There was a couple of things I had to do to resolve this.
One of the mistakes I made was putting the propellers on the wrong way.
I notice this when the back would lift and the front would not causing it to tilt forward. so double check they are correctly installed and your motor is turning in the correct way.

The next thing was to balance the quad by moving the battery towards the front of the electrohub until it balance. By lifting the quad with one finger on each side should lift evenly. My electrohub was back heavy when I would lift it so by moving it more towards the front it balanced it out.

The next thing I did to help is program the flip using the MultiWiiConf to auto level. I use the Aux1 to set the Angle mode. I plugged the flip to channel 5 on my receiver. Then I programed my Radio to use channel 5 for my landing gear switch to be able to turn it on or off by that switch on the radio.

As a beginner quad pilot the Angle mode really helps to control it better.

Also I was using a 4s battery in which it gave the quad a lot of power. Too much for me as a beginner. When I just give it a little power it would take off so fast and high.

One of the suggestion in the Quad for dummies video is to adjust the throttle in the MultiWiiConf called the "MID" the default was set to 0.50 I adjusted it down to 0.35.

The last thing is, the other wonderful people that replied to my post suggested not to give it slow power to take off. I agree,..
You need to give it enough power to get it to jump up in the air, about six feet or more then practice keeping it level.

Hope this helps.

Sidney

 

[joshuabardwell]

I could not keep the throttle at a certain speed to keep the quad level.
I tried to practice keeping it level 3ft off the ground it would drop and touch the ground. Then the propellers would speed up and shoot up in the air after it touched the ground.

Nothing is wrong. This is normal. As others have pointed out, you have to really be on the throttle at all times when flying a copter. With time, you will learn to anticipate what the copter is going to do and set the throttle appropriately. In the beginning, you are constantly responding to what the copter is doing, so you are always "behind the curve," if you will. This just takes time and practice.

As you mentioned, throttle expo in MW will help a lot with this. First set the expo to 1.0 (no expo). Next get the copter in the air and determine approximately what throttle setting it hovers at. You can also do this by holding the copter (CAREFULLY!) in one hand, level to the ground, and then slowly throttling up. At some point, you will feel the copter start to become weightless, and then with a little mroe throttle, it will start to try to take off. The "weightless" point is the hover point. Once you have determined the hover point, that is what you should set the MID value to. Don't just thrash around trying different values, because it will be hard to intuit what is right. So if your copter wants to hover at 40% throttle, set MID to 0.40.

Once you have set MID, next you can set the throttle expo. This will create a "flat" spot in the throttle response curve around the MID point. This means that the throttle will quickly "pop" up from zero, and then you will get more resolution as you are near hover. As you increase throttle to climb out, the resolution will drop again. I suggest starting with a throttle expo value of 0.5 or maybe even a little more. This is a relatively high value, but the thing is, a beginner will be spending most of their time hovering anyway. We want to quickly get off the ground, then hover. We don't care so much about making small, precise changes to throttle at the top and bottom of the curve. We need all the help we can get in hover, though.

Regarding the "pop" on landing, this is the gyro trying to compensate for the sudden change in the copter's attitude, due to hitting the ground. What I like to do is hover close to the ground, maybe 1-3 feet off the ground. Definitely out of ground effect. Then slowly reduce the throttle until I just start to see the copter start to descend. Don't change a thing. When the copter enters ground effect, it will stop descending and get all unstable. Just as the copter enters ground effect, or slightly beforehand if you can anticipate it, smoothly lower the throttle to zero. If you time this right, you will hit zero throttle just before the copter hits the ground, and it will set down nicely. If you are too fast, the copter will fall a bit further than you would like, but no big deal. If you do it too slowly, the copter will pop up again. With practice, you'll get the timing right. It's not actually too critical, and the copter can take a bit of a hard landing.

Also I did a gyro and acceleration calibration before flying

You seldom need to do a manual gyro calibration. The FC does a gyro calibration automatically on every power up. This is why it's important that you leave the copter absolutely still for a few seconds (until the LED stops flashing) after powering up.

I manage to break the ties on landing legs,..lol

That's what they're there for! Better to break the ties than to break a boom. Buy a pack and be ready to keep replacing them! I got tired of replacing zip ties, so I cut some sections of foam pool noodle and used velcro to attach them to my legs. They look kind of terrible, but they absorb hard landings much better. To each his own...

Regarding CG, if you have built the spider quad per FT instructions, the centroid of the motors' circle is as shown here:



If you want the CG to coincide with the centroid so that the motors work evenly, put it about here:

but when I try to disarm motor by moving the rudder all the way down to the left it does not disarm

Have you adjusted the channel center and endpoints on your transmitter so that they are 1000-1500-2000 in MWconf?

 

[SidneyKi]

Thanks Joshua for the great response.

 

[joshuabardwell]

Thanks Joshua for the great response.

No problem. I actually had those graphics already made from my build thread on my Electrohub.

 

[JourneyFPV]

THANKS!! I will give your suggestions a go and see how it works out. Having trouble with MultiWiiConfig reading ports on my Mac. might have to borrow a pc.

 

[Balu]

I still can't seem to get my head around

- center of lift (which should be in the crossing of straight diagonal lines between the motors if you use the same props, right?)
- center of the "motor circle"
- center of gravity

The flight controller (as in gyros) should be at the center of the motor circle?

COL and COG should be close to that point?

You can easily move the COG by moving the battery.

Changing the COL is a lot more difficult. I can think of two ways: changing the props or reducing it their efficiency - e.g. by turning them sideways - like on a v-tail. Both methods seem to be difficult to calculate, so we usually just ignore them.

I'm just brainstorming here for now, but I'd really like to understand it better.

 

[joshuabardwell]

I still can't seem to get my head around
- center of lift (which should be in the crossing of straight diagonal lines between the motors if you use the same props, right?)
- center of the "motor circle"
- center of gravity

I have never heard of the center of lift discussed in the context of a quad. In a fixed-wing, COL is important because if CG is behind COL, the plane becomes very unstable. Obviously, that doesn't apply in a quad. If the COL in a quad is dependent on the thrust developed by the motors, then it is constantly changing as the motors spin up and down. In short, I'm not sure how, or if, the concept of COL should apply to a multirotor.

Why do we care about the CG of a multirotor? One reason is that, if the CG is off-center, motors on one side will have to work harder than motors on the other side. This gives some motors less head-room to operate with, and means that the overall thrust that can be developed by the copter is limited. Consider: if the left motors have to spin at 60% to hover, and the right motors have to spin at 40%, then max thrust will be limited by the left motors. They will hit 100%, the right motors will hit 80%, and the right motors will not be able to spin up any more because the copter would begin to tilt.

By my thinking, this consideration means that the CG of the copter should be located at the centroid of the motors (center of the "motor circle") to allow all motors to run equally in hover, and to maximize the usable thrust that can be developed by the power system.

Now, what about the location of the FC? We know that the craft will tend to pivot about its CG. That's just Physics 101. So what happens if the FC is not located at the CG? Imagine that the FC is located all the way at the left side of the copter, near one of the motors. Now imagine that you roll to the left. The accelerometer in the FC will detect that the FC has dropped towards the ground and moved to the right. Take another example, where we yaw to the left. In this case, the accelerometer will detect that the FC has moved diagonally back and to the right. In short, when the FC is not located at the CG, then motions of the copter cause incorrect readings to occur in the accelerometer. The accelerometer is working correctly, but it's measuring the movement of one specific part of the copter, instead of the copter as a whole.

How does this matter? Well, first of all, if you're flying in Acro/Rate/Manual mode, I actually don't think it matters at all (someone please correct me if I'm wrong). The gyro will always read correctly, regardless of where it is located in the copter. (This ignores the issue of vibration isolation, which of course matters. Don't mount your FC at the end of one of your motor booms--duh.) Only the accelerometer will be "fooled" by mounting your FC away from your CG, and the accelerometer only comes into play when flying in an autolevel mode.

Let's elaborate on that. The FC is mounted at the end of the front-left motor boom. You yaw to the left. The accelerometer reports movement back and to the left. FC attempts to compensate by inputting right roll and front pitch. As soon as you stop yawing, the "false" accelerometer readings stop and autolevel works properly again.

Same example. You pitch forward. Accelerometer reports that the FC has moved down and backwards. FC inputs front pitch to compensate for backwards drift. If you are flying in Altitude Hold mode, then it may also throttle up to compensate for the downward movement. (My understanding is that most Alt-Hold algorithms use the accelerometer for small-scale tracking, and use GPS/Baro for elimination of accumulated error.)

What's the tl;dr? In my opinion, you should get your CG as close to the centroid of the motors as you can. But don't stress about this too much unless you are trying to maximize the maximum thrust that your copter can develop. If you seldom fly at high throttle settings, and if you seldom find yourself saying, "I need more thrust," then this is probably not a concern for you.

Likewise, you should try to place your FC as close to the CG as possible, but again, I think this seldom matters too much in real life. Most of the frames that copters use aren't large enough to get the FC far enough off-center to make it move very much. If you do some basic trig, you can calculate the length of the arc that the FC travels through when going from point A to point B on a circle of radius R, where R is the distance that the copter is from the CG. So if you imagine that the FC is 3" off of the CG and the copter rolls 10 degrees, the FC will have a "false" motion of about 1/2". Whether this ultimately matters depends on too many characteristics to list, but in many cases, it won't produce any noticeable adverse effect. Bear in mind, also, that the effect of the off-center FC will only be felt when actually inputting pitch/roll/yaw. As soon as the copter is stable (whether it's level or not), it will fly normally again.

Finally, I maintain (until proven wrong by someone smarter than me) that if you are flying in Acro/Rate/Manual mode, that FC placement doesn't matter, because the gyro is not affected by the "false" inputs caused by putting the FC away from the CG. Only FC orientation matters with the gyro (face forward) and vibration isolation.

 

[stay-fun]

I think you are quite correct, Joshua! For the gyros, they don't really care where they are mounted, because they only measure changes in angle, which is the same across the body of the model. For the accelerometers, all you say makes perfect sense, and I think you are right.

Now about center of lift, and center of gravity. In a hover, the CG and CL should be the same, wherever it is on the model. If the CG is for example shifted forward, the model will pitch forward (physics), and the FC will compensate and make the front motors spin faster. That changes the CL forward, until it's exactly at the CG.
Now the reason why CG is still important on a multirotor, is exactly what you already describe: certain motors have to work harder than others, and that messes with stabilisation/steerability. Let's take an extreme example, where the CG is so much forward that the front motors work at 90% in a hover, and the back at 10%. There is only little room for extra thrust for maneuvering and ascending the copter. Remember that maneuvering and stabilisation is done by letting the props spin faster/slower all the time, and at 90% power there isn't much headroom. Also at 10%, the props can't go much slower. That's why you want the CG close to the intersection of lines between opposing motors.

One nice example of a tricopter that's way nose-heavy is the episode with the lomo-copter.

 

[joshuabardwell]

What you seem to be saying is that CL is at the motor centroid when the motors' thrusts are equal, and the CL moves towards or away from a motor in proportion to the magnitude of the difference between that motor's thrust and the average of the other three motors' thrusts. When the CL is not under the CG, the copter tilts towards the CG. The FC adjust the CL by adjusting the motors' thrust to tilt the copter (or not) as desired.

This explanation supports the idea that the CG should be ideally located at the centroid of the motors' location, which should cause the motors to use equal thrust in hover, maximizing head-room.

Importantly--is it in fact the centroid (center of "motor circle") that is the correct location to cause equal thrust from the motors in hover? I have heard the "draw an x between the motors" approach, but I don't think it's right. I'm not sure why, though--it's just intuition.

 

[Balu]

Ok, we both agree that having everything a little off doesn't matter very much in the real world - unless you want to really optimize your copters performance. I also agree with your Acro theory. It sounds logical as long as just the gyros are involved.

But I'm not sure about the centroid.

If all motors are running at the same speed (and with the same props), isn't this "center of lift" in the intersection of the diagonals between the motors? This is where the center of gravity should be positioned?



If you put the center of gravity in the center of the circle, won't the quad tilt forwards in the right picture? So the two front motors would have to work harder to compensate?

 

[stay-fun]

Force on a certain point, is force (in Newton) times arm (distance between the point where the force acts upon, and the position between the actual force and this point)not sure I'm translating this right - Dutch is my native language). In a spiderquad, the picture on the right, the arm is larger for the front two motors, than for the rear two. I think the CG should be in the middle of the line that intersects the middles of the front two, and back two motors (if that makes sense). I can make a drawing when I get back home.

I think the whole circle thing is irrelevant.

http://en.wikipedia.org/wiki/Lever

 

[joshuabardwell]

Here's what I think is a giveaway, but I'm not going to draw it, so you'll have to imagine. Take an X-quad. The centroid and the center of the "X" are co-located. Move the front two motors outwards. As you do this, the circle gets larger and the centroid moves forward. Eventually, the centroid would no longer be inside the area circumscribed by the motors. It would be several feet in front of the center of the "X".

Obviously, this hypothetical "squished" quad could hover, but its CG could never co-located with its centroid. Therefore, it can't be the centroid that has to be co-located with the CG for hover to occur. It's probably the "X" then.

 

[joshuabardwell]

Ah, but, here's a question. This is a diagram of my ElectroHub Spider Quad:



And actually, I have since switched to an A-tail configuration with the rear motors even farther back and in. The center of the "X" between the motors is not on the frame. Whoops. Looks like my copter will never fly "optimally" whatever that means. Realistically, I'm hardly hurting for thrust. Nevertheless, this may be one good argument why "performance" copters should be symmetrical configuration--to minimize the thrust lost to having to compensate for off-center CG.

 

[RichB]

"Draw an X between the motors" works when the motors form a rectangle of some kind (or square). For shapes with non-parallel sides, like the trapezoid formed by a spider quad, you have to think differently.

Don't think of the motors as thrust sources, think of them as equal masses connected by rigid massless rods. The center of mass of the system will be the point at which the motors have equal lever moment on the system as a whole, and where you want your craft's CG to be to ensure equal thrust needed from each motor.

The thrust center (like center of mass if the motors were masses and not force sources) can be determined by recursively breaking the system down into pairs of objects and reducing each pair to a point between the two, then pairing those points off and finding their center.

 

[Craftydan]

Ahhhh . . . . the wonder of the "WEIGHTED" average.

If you pick a point on one axis (say, at the rear motors along the roll axis) as your origin and average the distance along that axis, then measure and run a weighted average on the motor's distances (recognizing that some could be negative distances depending where you pick the origin), then you'll come back to the same point . . . no recursion involved, and works for *any* number of motors.

Additionally, with the same motors/props, the weights are all the same and can be ignored. with something like a V-tail, you can lighten their weight by the sine of the tilt angle, to adjust for the thrust lost.

 

[joshuabardwell]

Ahhhh . . . . the wonder of the "WEIGHTED" average.

All the more reason to just fly in acro mode and be done with, if you ask me! All this math and geometry and nonsense...

 

[Craftydan]

All the more reason to just fly in acro mode and be done with, if you ask me! All this math and geometry and nonsense...

Actually . . . putting your CG in the wrong spot will make it fly equally bad in an acro or a self-level mode. More so, if the board is off the CG, but even if it's on the CG the motors won't care if the board is keeping it level or you are when two are taking it easy and the other two are struggling to keep balance.

Math is good . . . but yeah, it can make the head hurt